The present invention relates to an optical scanning device in use with an image recording apparatus which records an image on a recording medium by scanning a surface of the recording medium with a laser beam containing image information.
Conventionally, an optical scanning device which employs a divisional scanning method based on an overfilled optical system in order to satisfy the market demands of increasing the operation speed and resolution performances of the image processing apparatus and to obtain a wide scan width without increasing the focal distance (optical path length), which is shown in Unexamined Japanese Patent Publication No. Hei 10-177147.
The proposed optical scanning device is basically composed of two light source members 40A and 40B, two first optical systems 42A and 42B respectively provided for the light source members 40A and 40B, a polygon mirror (multi-faceted rotating polygon mirror) 12 which receives laser beams modulated by an image signal and repeatedly deflects the laser beams every line of the image signal, and a single second optical system 48 which substantially uniformizes the scanning speed of the deflected laser beam over a photosensitive body 46 (surface to be scanned) and focuses the laser beam at a position near the photosensitive body 46.
The optical scanning device is designed so as to satisfy the following condition ##EQU1## provided that a scan angle by the deflection is .+-.2.alpha.; the incident angles of the two beams when incident on the same facet of the polygon mirror 12 are -.alpha. and +.alpha. in the main scanning direction with respect to a center line CL connecting the rotational center of the polygon mirror 12 to the center position on the photosensitive body 46; a rotation angle of the polygon mirror 12 is equal to the incident angle of the two laser beams, ##EQU2## and the number of facets of the polygon mirror 12 is n.
Of the two laser beams, the laser beam A (indicated by a dotted line) scans a range from 1 to 2 at a scan angle of -2.alpha., while at the same time the laser beam B (indicated by a solid line) scans a range from 3 to 4 at a scan angle of +2.alpha..
This double-beam and surface-division scanning method succeeds in suppressing such a phenomenon essential to the surface-division scan based on the overfilled optical system; the uniformity of the beam diameter on the surface of the photosensitive body 46 is deteriorated in linking with a variation of the F-number (equivalent to "brightness" in the field of the camera) and the beam diameter difference appears at the central position of scanning.
As seen from FIG. 6, where the overfilled optical system is used, the width (Do) of the bundle of rays of incident light is larger than the width of the facet of the polygon mirror 12. Therefore, the light beams incident on the facet a preceding to the reflecting facet b (facet a: located on the mirror rotation start side) and the facet c succeeding to the same (facet c: located on the mirror rotation end side) are reflected by those facets, and the reflected ones are unnecessary light N.
As shown in FIG. 1, the laser beam A scans the range from 1 to 2 while at the same time the unnecessary light N.sub.A scans the range from 3 to 4. The laser beam B scans the range from 3 to 4 while at the same time the unnecessary light N.sub.B scans the range from 1 to 2. Thus, the unnecessary light doubly writes its image on the photosensitive body surface having an image already written therein.
To avoid this double writing problem, as shown in FIGS. 7 and 8, there had been proposed an optical scanning device in which the laser beams A and B are incident on the surface of the photosensitive body 46 at different incident angles .beta.1 and .beta.2 in the sub scanning direction, and reflecting mirrors 50A and 50B are provided for the laser beams, respectively, which is shown in Unexamined Japanese Patent Publication No. Hei 10-206761.
With this optical arrangement, the laser beams deflected by the polygon mirror 12 at different angles in the sub scanning direction, and those beams are sufficiently spaced from each other in the sub scanning direction at a position where the reflecting mirrors 50A and 50B are disposed. Therefore, there is no chance that the unnecessary light N.sub.A and N.sub.B hits the reflecting mirrors not associated with these beams of unnecessary light.
Specifically, the unnecessary light N.sub.A (indicated by dashed line) of the laser beam A does not hit the reflecting mirror 50B provided for reflecting the laser beam B and is not reflected by the same. Similarly, the unnecessary light N.sub.B of the laser beam B does not hit the reflecting mirror 50a provided for reflecting the laser beam A and is not reflected by the same. Therefore, the double writing problem can be avoided.
However, this approach encounters another difficulty to secure formation of the dot at the central position of scanning (joining point or position between the laser beams A and B).
This will be briefly described with reference to FIG. 9. Where the beam diameter .omega..sub.0 is several tens .mu.m on the photosensitive body 46 in the main scanning direction, the beam diameter is several mm on each of the reflecting mirrors 50A and 50B that are disposed on the optical paths ranging to the photosensitive body 46. As a result, each reflecting mirror fails to receive some part of the laser beam, and the laser beam of an imperfect beam diameter appears at the central position of scanning. In this case, the dot formed is deformed in its shape.
Accordingly, an object of the present invention is to provide an optical scanning device which solves the problem of the double writing by unnecessary light and secures the formation of the dot in the vicinity of the central position of scanning, even if the overfilled optical system is used.